Disk drives are utilized in many information and processing systems for storing data. A typical disk drive includes a spindle motor for rotating a data disk, and an actuator for moving a head carrier that supports read/write heads radially across the disk to access data stored on concentric data tracks on the disk. Many disk drives include a plurality of disks separated by spacer rings and stacked on a hub attached to the spindle motor, a plurality of read/write heads, and a plurality of of head carriers, each head carrier supporting at least one head. A head carrier typically includes an air-bearing slider that rides on a bearing of air above a disk surface when the disk is rotating at its operational speed.
A data disk surface is typically very smooth in order to increase storage capacity of the disk and to maintain consistent magnetic properties across the disk. However, a smooth disk surface causes stiction when a head carrier slider comes into stationary contact with the disk surface. Stiction causes the slider to stick to the disk surface, resisting rotation of the disk.
Stiction can be caused by static friction, and adhesion forces between the disk surface and the slider created by the lubricant on the disk surface. Stiction has a step function characteristic with a maximum value when the disk and the slider are in stationary contact and a minimum value when the disk and the slider are moving relative to one another. Therefore, when disk rotation is initiated, stiction between the slider and the disk instantly drops from the maximum value to the minimum value, causing the slider to suddenly break free of the disk. Further, in many low power disk drives such as used in laptop computers, despite application of full power to the spindle motor, the spindle motor cannot overcome stiction between the slider and the disk. In that case, the disk drive is not operational and none of the data on the disk can be accessed.
To prevent the slider from sticking to the disk, some disk drives, such as contact start/stop disk drives, provide a parking zone for the slider, away from the smooth surface of the disk. In contact start/stop disk drives, the slider is in contact with the disk surface during start and stop operations when there is insufficient disk rotational speed to maintain an air bearing between the slider and the disk surface. The disk includes a landing zone in a specially textured non-data region of the disk where the slider is parked during stop and start operations. In other disk drives, the slider is mechanically unloaded from the disk when the disk is not operating, and then loaded back on when the disk has achieved sufficient rotational speed to provide an air bearing for the slider. To unload the slider, a ramp is included in the disk drive housing to contact the head carrier when the head carrier is rotated to the outer periphery of the disk. The ramp lifts and suspends the slider off the surface of the disk.
However, the slider can come into stationary contact with, and stick to, the disk when, for example, the slider is displaced from the landing zone or the ramp due to external shock or actuator failure. If the spindle motor cannot overcome stiction between the slider and the surface of the disk to rotate the disk, none of the data stored on the disk can be accessed. The disk must be physically removed from the disk drive and the date stored thereon accessed by other means.
To start the disk rotating, some disk drive manufacturers have incorporated a dithering function into the actuator controller. The actuator controller applies a train of short duration pulses to the actuator and dithers the head carrier radially to gradually break it free of the disk. Specifically, at startup, a normal startup routine attempts to rotate of the disk. If the normal startup routine fails to rotate the disk, an emergency startup routine is initiated where power is applied to the spindle motor and the head carrier is dithered for a period of time to break it free of the disk. If at the end of the dithering period the disk is rotating, the emergency routine ends. Otherwise, the procedure is repeated until the disk begins rotating or a timeout occurs.
A problem with such disk drives, however, is in the way power is applied to the spindle motor to start the motor and accelerate it to its operational speed. A typical disk drive spindle motor is a brushless DC motor having multiple phase windings arranged as a stator, and a rotor having a permanent magnet for rotating the disk. During an acceleration phase, the motor is commutated to start from standstill and accelerated to its operational speed. Thereafter, the motor is commutated to maintain that operational speed, by sequentially energizing appropriate phase windings based on the location of the rotor to the phase windings. The energized windings generate torque inducing magnetic fields relative to the rotor magnet that rotate the rotor.
However, in such disk drives, the position of the rotor relative to the phase windings at startup is not known. Frequently, inappropriate phase windings relative to the rotor are energized, generating magnetic fields that induce little or no torque to rotate the rotor. As a result, despite application of full power to the spindle motor and dithering of the head carrier, the motor cannot overcome stiction between the slider and the surface of the disk. Further, energizing inappropriate phase windings relative to the rotor can generate magnetic fields that turn the rotor and the disk in the wrong direction, damaging the slider and the read/write heads.
In order to ensure that proper phase windings are energized, sensing devices, such as Hall sensors, can be used to determine the rotor position relative to the windings at startup. Once the rotor position is determined, appropriate windings can be energized to provide maximum torque to the rotor. The rotor position can also be used to energize appropriate windings to rotate the rotor in the correct direction every time. However, such sensors must be precisely positioned and the sensor components are expensive and take up physical space in the motor. Further, environmental variations such as temperature fluctuations severely affect their accuracy.
Some disk drives include indirect position detection systems, such as back electromotive force (emf) detectors, that do not require sensors. Back emf detectors sense back emf transitions in the phase windings, due to magnetic flux caused by a moving rotor, to identify the proper phase windings to be energized. However, such detection systems do not provide rotor position information when the rotor is stationary relative to the windings or when the rotor is not rotating at or above a certain speed. For example, when the rotor is not rotating due to stiction between the disk and the slider, there are no back emf transitions in the stator windings to hint at the rotor position.
In order to detect rotor position at startup, some disk drives include a detector for injecting short current pulses in different phase windings. The times required for the currents in each phase to reach a predetermined level are measured and subtracted from one another. The magnitude and sign of the results are then utilized to determine rotor position relative to the windings at startup. Thereafter, appropriate windings are energized by open loop commutation of the motor based on timing tables pre-stored in memory. The timing tables include information for energizing selected windings at the most beneficial times and for providing torque to the rotor in the correct direction.
However, a problem with such drives is that timing of the commutations is based on expected rotational positions of the rotor relative to the windings. In particular, after the rotor position is initially determined and the appropriate phase windings are energized to start rotation of the rotor in the correct direction, the timing of the commutations is based on the predetermined timing tables. Therefore, if the rotor does not rotate to the next commutation point in a predetermined time, as expected, the commutation timing will be off. Thereafter, the phase windings energized for commutating the rotor may not provide torque to the rotor at the most beneficial time. Improper commutation timing can impede rotation of the rotor, slow the rotor to a stop, and cause irregular rotor rotation speed. This is particularly problematic where stiction between the head carrier slider and the smooth surface of the disk, impedes rotation of the disk at startup. In that case, the time necessary for the rotor to rotate to the next commutation point is vastly different from that predicted by the timing tables.
A further disadvantage of open loop commutation is that a commutation timing table developed for a particular disk drive may be unusable for another disk drive with different physical characteristics. For example, the number of disks in a disk drive directly affect the frictional and inertial forces present in the disk drive. The friction forces in turn affect the rate of rotation of the rotor, requiring a different commutation timing. As such, disk drives with differing number of disks require differing commutation timing tables, increasing the cost and complexity of all disk drives. The commutation timing tables also take up precious memory space, such as ROM.
Other disk drives include rotor position detectors for applying short duration current pulses to each motor phase and monitoring the motor current responses to each pulse. Typically, the amplitude of one of the motor current responses is higher than the rest, and the motor phase producing the higher amplitude indicates rotor position. However, a problem with such drives is that the difference between the pulses returned from the different phases may be very small. As such, determining rotor position based on the differences is difficult and unreliable. Further, measurement accuracy is affected by temperature and differences between the phase inductances or phase resistances.
There is, therefore, a need for a disk drive capable of overcoming stiction between head carriers and one or more disks, and accelerating the disks from standstill in the correct direction, while reliably and consistently applying maximum forward torque to the disks.